1. Field of the Invention
The present invention relates generally to rolling materials such as metal sheets and, more particularly, to a rolling mill having crossing work rolls and exhibiting excellent sheet crown control performance. The invention also is concerned with a rolling method employing the rolling mill and further to a method of using the rolling mill.
2. Description of the Related Art
One of the important criteria for evaluating the quality of rolled sheet is the sheet thickness distribution in the direction of breadth of the rolled sheet, i.e., sheet crown. The sheet crown varies according to rolling conditions or factors such as deflection of rolls due to rolling load, thermal crown of the rolls, wear of rolls, and so forth.
Hitherto, various measures have been considered to compensate for the influence of the above-mentioned factors on the sheet crown so as to make it possible to produce rolled sheets of desired profiles. One of such measures is to arrange rolls in a crossing manner so as to vary the vertical gap profile between work rolls or between each work roll and a cooperating back-up roll.
Known roll cross rolling methods are theoretically sorted into the following three types:
(1) Crossing arrangement is employed only for work rolls (Disclosed in, for example, M. D. Stone: Iron & Steel Engineering, August 1965) PA0 (2) In the case of a 6-high mill, crossing arrangement is employed for back-up rolls or intermediate rolls which are in support of work rolls (Disclosed in, for example, M. D. Stone: Iron & Steel Engineering, August 1965) PA0 (3) Roll pairs, each including a work roll and a back-up roll, are arranged to cross each other (Disclosed in, for example, Japanese Patent Publication No. 58-23161) PA0 (I) Relative slippage of rolls S PA0 (II) State of lubrication between rolls .mu..sub.o PA0 (A) Force generated due to crossing between the work roll and the back-up roll PA0 (B) Force generated due to crossing between the work roll and the rolled sheet and acting in the direction counter to the force (A) above.
The method (1) has been tested and examined by Kono et al as reported in a literature "Spring Session of Plastic Work", Showa 56 (May, 1981), while the method (2) has been tested by A. R. E. Singer et al. as reported in a literature "Journal of the Iron and Steel Institute", December 1962. These methods, however, proved to be impractical due to too large thrust forces generated for given rolling load due to crossing between the work rolls and the back-up rolls. In fact, the method (1) showed a high level of thrust ranging from 8 to 13% the rolling load. Large thrust, say 6.5% the rolling load, was observed in the method (2).
In view of these problems, the method (3) has been proposed as roll cross rolling method which can vary the profile between the work rolls without generation of thrust between the work roll and the back-up roll. As stated before, according to this method, pairs of rolls, each pair including a work roll and a cooperating back-up roll, are arranged to cross each other. In this case, the thrust acting on the work roll is generated between this work roll and the sheet, so that the level of the thrust is about 6% at the greatest even in the region where the rolling reduction or draft is as high as 40%. Such thrust can be borne by bearing structures which can be realized in spaces restricted by the upper and lower roll pairs each including the work roll and the back-up roll.
The rolling mill realizing the method (3) described above, generally referred to as a "pair cross mill" suffers from a disadvantage in that back-up rolls of heavy weights are to be moved. In addition, since the position of the back-up roll relative to the draft screw which bears the rolling load varies due to the movement of the back-up roll, a moment or couple of force is applied to a back-up roll chock, causing a tilt of the chock or uneven contact between the chock and the housing. In order to overcome this problem, a cross beam is provided between the back-up roll chock and the draft screw or between the back-up roll chock and the housing. Consequently, the whole construction is rendered complicated. The construction is further complicated due to the necessity of suitable anti-friction means such as plain bearings interposed between the cross beam and the draft screw of between the cross beam and the associated portion of the housing, in order that the work roll and the back-up roll are moved smoothly with reduced sliding resistance.
Japanese Unexamined Patent Publication No. 5-50110 discloses a rolling mill which has a simple construction but yet capable of performing crown control, thereby overcoming the above-described problem. This rolling mill is of the type in which only the work rolls are arranged to cross each other in the rolling method type (1) stated before. In this rolling mill, however, the thrust generated due to crossing between the work roll and the back-up roll is reduced to a level of 4 to 10% the rolling load, by lubricating effect offered by a liquid mixture (emulsion) of a base oil such as a mineral oil or a beef tallow and water containing 0 to 10% of oil.
In this rolling mill, the thrust is transmitted from the back-up roll to the work roll. The work roll also is subjected to thrust which is generated between the work roll and the sheet. This thrust acts in the counter direction to the thrust imparted by the back-up roll.
These two kinds of thrust cancel each other, so that the thrust applied to the work roll is actually 0 to 4%, 5% at the greatest, of the rolling load. It is therefore possible to bear against this thrust by a bearing or the like structure which is disposed in a limited space between the work roll and the back-up roll of the cooperating pair.
Thus, a rolling mill having work rolls crossing each other, which hitherto has been considered to impossible to practically realize, can be obtained by creating suitable conditions of lubrication between the work roll and the back-up roll.
It is to be pointed out, however, the inter-roll friction coefficient, which is one of the major parameters of the lubrication conditions, is largely ruled by the roll rolling conditions.
More specifically, the lubrication is performed by a film of the lubricating oil which is tapped into the nip between the rolls as a result of the roll rotation and which reduces the friction coefficient. The effect of reducing the friction coefficient is drastically increased when the roll rotation speed is increased and the friction coefficient is finally set to a range of from 0.04 to 0.12, although it varies according to the type of the lubricating oil used. When the roll rotation speed is low, however, the friction coefficient is large, say 0.15 or greater, as shown in FIGS. 2 and 3, allowing generation of large thrust.
When the cross angle of the work roll (angle to the direction perpendicular to the pass line) is increased, the thrust applied by the back-up roll to the work roll is saturated when the cross angle exceeds 0.5.degree., whereas the thrust imparted by the rolled sheet to the work roll is increased in proportion to the increase in the cross angle.
Therefore, the composite thrust acting on the work roll, as shown in FIG. 1, first appears to act in the direction of the thrust imparted by the back-up roll and this thrust exhibits a peak when the cross angle ranges between 0.5.degree. and -1.0.degree.. This thrust then starts to decrease and, when the cross angle exceeds 1.5.degree., the thrust acting in the counter direction becomes dominant. The thrust acting in the counter direction increases in proportion to the increase in the cross angle and finally exceeds the level which can be borne by the work roll. This fact has been confirmed also through experiment, as will be seen from FIG. 4.
The limit of the thrust which can be borne by the work roll depends on the thrust-bearing capacity of the bearing incorporated in the work roll chock.